1. Field of the Invention
Recovery of heat, energy and chemicals from pulp mill liquor.
2. Description of the Prior Art
A number of patents disclose drying the spent liquor solids and burning the dried liquor solids in a recovery boiler. Exemplary are U.S. Pat. Nos. 1,779,768 and 2,406,581.
It has also been proposed to substitute a pyrolysis and gasification unit for a recovery boiler. Patents disclosing this arrangement are: Brink et al, U.S. Pat. Nos. 3,639,111, 3,718,446 and 3,761,568; and Holme, U.S. Pat. No. 3,867,251.
A number of patents disclose hydropyrolysis of spent liquor. This process yields a wet coke and an aqueous solution of inorganic chemicals. These are U.S. Pat. Nos. 3,558,426; 3,591,449; 3,595,742; 3,607,619; 3,705,077; 3,855,069; 3,884,751; and 3,762,989.
In a modern Kraft Pulp Mill, the chemical recovery boiler represents the single most expensive item of capital investment. Black liquor from pulping and brown stock washing is evaporated to 55-65% solids and sprayed into the boiler. Here it is burned to recover heat and pulping chemicals.
The organic materials in a typical kraft black liquor will normally comprise about 55% of the solid material present. The heating value of the dry solids will be approximately 6600 BTU/lb. Liquor, as ordinarily sprayed into the recovery boiler at 60-65% solids content, will have a net heating value about 3700 BTU/lb.
In this application, the terms recovery boiler, recovery furnace, chemical recovery boiler and recovery unit are used synonymously and refer to technology well established and understood in the pulping industry.
The ultimate capacity of a recovery unit, if undesirable particulate and gaseous emissions are not considered, is limited by the amount of heat transfer surface in the boiler section. About 10% of the heat value of a 65% solids liquor is lost as a result of the residual water. It has been proposed to increase the capacity of recovery boilers by this amount by the expedient of using wholly dried black liquor as a feed material. Experience has shown that overall thermal efficiency of the recovery boiler does increase. However, little or no added capacity is realized in terms of pounds of additional liquor solids that can be processed in a given unit of time.
In a conventionally operated recovery boiler, the concentrated black liquor is sprayed into an upper, oxidizing zone. Here the residual water is evaporated and a partial combustion of the organic materials occurs. Turbulence is high in this section of the furnace and it is here that there is a considerable entrainment of particulate matter into the flue gases. The partially burned material falls onto a bed on the hearth area of the furnace where a reducing atmosphere prevails. The bulk of the remaining organic materials is converted to carbon monoxide which is burned as it rises into the upper oxidizing zone. The conversion of the sodium sulfate in the black liquor back to sodium sulfide occurs in this bed. A blanket of partially burned liquor lying on the surface of the bed protects the system against reoxidation. The resulting inorganic residue is drawn off the bottom of the bed as a molten smelt and used for the makeup of cooking liquor.
An excellent treatment of the subject is found in the book "Chemical Recovery in Alkaline Pulping Processes," TAPPI Monograph No. 32 (1968) and there is no need to repeat all the details in this application.
There has always been some loss of inorganic materials in fine particulate form associated with the recovery boiler. Additionally, there have been malodorous gases formed from the sulfur-containing components of the liquor.
In the past, emissions of solids and of sulfur-containing gases have generally been of acceptable levels when the recovery boiler was operated at or below its design capacity. In the event of an expansion which added additional pulping capacity to the mill, it has usually been more acceptable to run the boiler in an overloaded condition than to replace it with a new boiler of larger capacity. Depending on the size of the mill, recovery boilers today cost approximately $20-30 million. Overloading the boiler tends to result in marked increases in the emission of particulate matter and the obnoxious sulfur-containing gases. In view of pollution regulations which are becoming increasingly stringent, it is no longer acceptable, either legally or socially, to run recovery boilers in this condition.
Obviously, because of the high cost, replacement of a recovery boiler is not a matter to be taken lightly. In a number of cases, particularly in older mills which may have been running at marginal profitability, the only reasonable alternative has been to close the mill. This, of course, has a serious and adverse financial impact on both the employees and the community in which the mill is located.
The dilemma between closing a mill or replacing the recovery boiler with one that will meet current and expected future pollution requirements has prompted a search for new recovery processes. The references cited in this application are all directed toward the solution of problems of this general type. It has been pointed out earlier that the use of wholly dried liquor as a feed to the recovery furnace is no answer to the mill with an overloaded boiler.
One possible alternative is in the early development stage and is as yet a long way from being ready to put into a full sized pulp mill. These are units typified by the previously mentioned patents to Brink, et al, in which the concentrated black liquor is subjected to a one or two stage pyrolysis-gasification process. Here the black liquor is typically pyrolyzed in a first zone to a char and a fuel gas. This char, which still contains most or all of the inorganic materials, then drops into an oxidizing zone where the carbon is burned off and the inorganic materials recovered. The process, as now conceived, is not well designed for kraft liquors since a very indirect route must be followed to regenerate the sodium sulfide from the sodium sulfate found in the black liquor. Equipment for this entire process is not presently available in the market place. Even if it were, it would still involve major capital expenditure because the existing chemical recovery boiler would be totally replaced.
The comments made above in regard to the proposed pyrolysis-gasification process apply equally to a second alternative which is based on hydropyrolysis of the spent liquor.
It is clear that none of the above processes deal with the question as to how a mill can make advantageous use of its in-place recovery boiler and still meet pollution standards without the expenditure of huge amounts of capital. It is to this end that the present invention is directed.